a. Field of the Invention
This invention relates to data compression systems and more particularly to such a system which employs a two-dimensional fast transformation, of a type such as the Haar transformation, to reduce the bandwidth required for the transmission of two-dimensional video data.
b. Description of the Prior Art
U.S. Pat. No. 3,981,443 describes a class of transforms having certain properties which are desirable for the processing of video data in real time. The class of transforms, denoted herein as "fast transforms", has the property that each transform may be expressed as a cascade of elementary transformations in which each elementary transformation is composed of the product of a diagonal weighting matrix and a sparse matrix composed of +1, and -1 and zero elements. By virtue of the decomposition of the transformation into the product of elementary transformations, any member of the class of transforms can be implemented by, at most, 2 Nlog.sub.2 N real computations, where N is the dimension of the data vector and is an integral power of 2. U.S. Pat. No. 3,981,443 describes means for mechanizing such fast transforms which require only a relatively small number of adders, subtractors, delayers and multipliers, and which has the ability to achieve high data rates. U.S. patent application Ser. No. 673,799 filed Apr. 5, 1976, and assigned to Northrop Corporation, the assignee of the present application, describes the application of a member of the class of fast transforms, the one-dimensional Haar transformation, to video data for the purpose of bandwidth reduction. In the system of this prior patent application, transform coefficients of a size less than a specified threshold level are eliminated and only the remaining significant coefficients are transmitted, thereby achieving a reduction in bandwidth. In the aforementioned prior application the threshold level in the one-dimensional system is set by the operator so as to transmit all transform coefficients larger than a specified size or to transmit only the "n" largest coefficients. There is no provision for the automatic adjustment of the threshold level to take advantage of variations in the number of significant coefficients which can occur in successive arrays of video data. Also no provision is made in the aforementioned prior application for the detection and correction of errors in the received data.